For a slip event at interface $s$, we have $\Delta R_s > 0$ and $\Delta R_i = 0$ for $i \neq s$, inducing
\begin{equation}
	\label{eq:delta_fi}
	\Delta f_i = K\Delta R_s \,\, \mathrm{for} \,\, i \geq s ; \quad \Delta f_i=0 \,\, \mathrm{otherwise}.
\end{equation}
We can then deduce that
\begin{equation}
	\label{eq:DR_DF_multi}
	\Delta F = \frac{h K}{H} \Delta R_s \sum\limits_{i=s}^n i \, = \frac{h K}{H}\Delta R_s (n+s)(n-s+1) / 2.
\end{equation}
which we verify in \cref{fig:2c}.
